A threaded joint for steel pipes is constituted by a pin, which is a component which has male threads (external threads) and which is provided on the end portion of a first tubular member, and a box, which is a component which has female threads (internal threads) and which is provided on the end portion of a second tubular member. Connection of the joint is carried out by threaded engagement of the male threads and the female threads, both of which are typically the tapered threads. Typically, the first tubular member is a pipe such as an oil country tubular good, and the second tubular member is a separate member in the form of a coupling. (This type of threaded joint for steel pipes is referred to as a coupling type). In this arrangement, a pin is formed on both ends of a pipe, and a box is formed on both sides of a coupling.
Another type of threaded joint for steel pipes is the integral type which does not use a coupling and which has a pin formed on the outer surface on one end of a pipe and a box formed on the inner surface of the other end of the pipe. In this type of a threaded joint, the first tubular member is a first pipe, and the second tubular member is a second pipe.
In the past, oil country tubular goods were connected primarily using standard threaded joints prescribed by API (American Petroleum Institute) standards. However, in recent years, as the environments for excavation and production of crude oil and natural gas are becoming severe, special high performance threaded joints referred to as premium joints are increasingly being used.
In a premium joint, each of the pin and the box has, in addition to tapered threads which enables the joint to be tightened, a seal surface provided on its peripheral surface in the vicinity of the threads and a shoulder surface which serves as an abutting stopper during makeup of the joint. The joint is designed to allow radial interference between the seal surfaces of the pin and the box. If the joint is tightened until the shoulder surfaces of the pin and the box abut each other, the seal surfaces of these members intimately contact each other around the entire periphery of the joint and form a seal by direct metal-to-metal contact. The shoulder surfaces not only act as abutting stoppers at the time of makeup but also act to bear a compressive load acting on the joint.
FIG. 1 is a schematic explanatory view of a coupling type threaded joint for steel pipes of the typical premium joint type. (A) is an overall view, and (B) is an enlarged view of a portion thereof. As shown in FIG. 1(B), the threaded joint for steel pipes has a pin 1 which is a component with male threads provided on the end of a pipe and a box 2 which is a corresponding component with female threads provided on both sides of a coupling. On its outer surface, the pin 1 has tapered male threads 11 and an unthreaded cylindrical abutting portion called a lip (referred to below as a lip portion) 12 provided on its tip adjoining the male threads 11. The lip portion 12 has a seal surface 13 on its outer peripheral surface and a (torque) shoulder surface 14 on its end surface.
The opposing box 2 has tapered female threads 21, a seal surface 23, and a shoulder surface 24 which can engage with or abut the tapered male threads 11, the metal seal surface 13, and the shoulder surface 14, respectively, of the pin 1. Some of the female threads at the tip of the box (in the illustrated example, four threads) are non-engaging threads which do not completely engage with a thread of the pin. In this case, some of the corresponding male threads at the inner end of the pin are incomplete threads which do not have the desired male thread shape required for complete engaging with a female thread. The presence of incomplete threads in this location in either a pin or box or both is necessary for smooth insertion of the pins at the time of makeup of a joint. In some joints, the male threads on the tip side close to the seal surface of the pin are incomplete threads which do not engage with the female threads of the box.
As shown in the figure, a lip portion which has a shoulder surface as an end surface is typically provided on the tip of the pin, but it is also possible to provide a lip portion on the tip of the box or to provide it on the tips of both the pin and the box.
FIG. 2 is a schematic view for explaining the shape and dimensions of a trapezoidal thread of which an API buttress thread is typical. As in FIG. 1, 11 is a male thread and 21 is a female thread. Threads used in premium joints are mostly trapezoidal threads emulating this API buttress thread. With many threads, the aspect ratio of the threads (ratio of height to width), the flank angles (the angles of slope of the side surfaces or flanks), and the like almost exactly copy the dimensions of an API buttress thread.
In FIG. 2, in the case of API buttress threads having a thread pitch of 5 TPI (5 threads per inch), the thread height 74, which is the height of the crest of the male threads, is 1.575 mm, the flank angle 71 (loading flank angle) of the loading flanks (which are the side surfaces of threads on the rear side in the direction of insertion of a pin) is 3°, the flank angle 72 (stabbing flank angle) of the stabbing flanks (which are the side surfaces of threads on the front side in the direction of insertion of a pin) is 10°, and the average value of the separation 73 in the axial direction of the joint 73 between the stabbing flanks of the male threads and the female threads (the stabbing flank gap) is approximately 100 μm (30-180 μm).
When vertical wells were predominant, a threaded joint for steel pipes could adequately function if it could withstand the tensile load due to the weight of pipes connected to it and prevent leaks of high pressure fluids passing through its interior. However, in recent years, in light of the fact that wells are becoming deeper and sloping wells or horizontal wells having a well bore which is curved underground are increasing and the development of wells in worse environments such as in the oceans or polar regions is increasing, threaded joints are demanded to have a wide variety of properties including resistance to compression, bending resistance, sealing performance against external pressure, and ease of use in the field.
Concerning the thread shape of a threaded joint for steel pipes, below-listed Patent Document 1 describes a threaded joint for steel pipes in which the threads of both a pin and a box, i.e., both the male threads and female threads are given a two-step stabbing shape having a chamfer made by removing a region between the crest of a thread and the stabbing flank along a straight line or a curve. The chamfered portion functions as a contact surface which is the first region to undergo contact when the pin is inserted into the box. The purpose of this contact surface is to facilitate insertion by contact of the contact surfaces of the pin and the box if the pin and the box are misaligned in the axial direction when the pin is being inserted into the box.
Below-listed Patent Document 2 describes a similar pipe joint. Namely, a corner chamfer is provided on the stabbing flanks of the threads of both a pin and a box. When the pin is inserted into the box, the corner chamfers engage with each other and facilitate insertion of the pin.
Both of Patent Documents 1 and 2 prevent misalignment of the insertion angle and facilitate insertion by producing contact between a pin and a box in chamfered portions between the stabbing flanks and the crests. Accordingly, chamfers are necessary on both the pin and the box, and the intended effect is not exhibited if chamfers are provided on only one of the two members.
In below-listed Patent Document 3, a threaded joint for steel pipes named “cylindro-conical pipe joint” is disclosed. It has complete threads in a cylindrical threaded zone and incomplete threads in a frust-conical threaded zone. For the box, a two-step stabbing shape in which a 45° bevel having a height of approximately one-half of the thread height is formed in the stabbing flanks of only the incomplete threads, thereby facilitating insertion of the pin. However, such a large beveling (chamfering) is not carried out with respect to the complete threads where the male threads of the pin engage the female threads of the box. Furthermore, the threaded joint disclosed in this document is designed for parallel threads, not tapered threads.
In below-listed Patent Document 4, as shown in FIG. 3 attached hereto, a threaded joint for steel pipes having a nose portion 15 provided between a seal surface 13 and a shoulder surface 14 at the end of a pin 1 is proposed. The nose portion 15 of the pin 1 does not contact the opposing portion of the box 2. On the other hand, the seal surfaces 13 and 23 and the shoulder surfaces 14 and 24 of the pin and the box contact each other. By extending the lip portion of the pin and providing a non-contacting nose portion 15 beyond the seal surfaces, the wall thickness of the lip portion in a limited pipe wall thickness and accordingly the wall thickness of the shoulder surfaces and the seal surfaces can be increased, and the resistance to compression of a threaded joint for pipes and its ability to seal against external pressure can be markedly increased.
Patent Document 1WO 92/15815Patent Document 2U.S. Pat. No. 6,322,110Patent Document 3U.S. Pat. No. 4,398,756 (FIG. 5)Patent Document 4WO2004/109173